Do Leaf's Batteries Really Have Twice The Energy Density?

Nissan is saying that its new lithium-ion battery in the Leaf EV has doubled its old energy density but, so far, the numbers don’t bear that out.

When we interviewed several Nissan engineers for our June cover story on the Leaf, all told us that the battery’s energy density had doubled since the days of the company’s Altra EV more than a decade ago. We’ve seen the same claims made repeatedly in various news stories and blogs.

So we checked. A Design News feature story from October 5, 1998 said of the Nissan Altra EV: “At 90 Wh/kg, it reportedly provides the Altra with a 120-mile driving range.”

When we interviewed Nissan engineers in mid-April, we asked whether the new energy density was 180 Wh/kg (which would have been twice that of a decade ago). Nissan executives and engineers told us they weren’t yet able to discuss hard numbers, but said it was roughly in that neighborhood.

In an effort to nail it down a little tighter, we then called experts: David Swan of DHS Engineering (who designed the battery for the world’s fastest EV a decade ago); Donald Sadoway of MIT; Donald Hillebrand of Argonne National Labs; and Jacob Grose, an analyst for Lux Research. Providing them with gross battery weight and a few other parameters, we asked for opinions. All were surprised by the 180 figure; two were outright doubtful of its accuracy.

More recently, the actual numbers have come out, and the real energy density turns out to be 140 Wh/kg, per Nissan’s media web site. That’s about 55% higher (not twice as high) as a decade ago.

Perhaps the most insightful interpretation of the situation came from Swan (before the actual numbers were made public): “I suspect that the ‘factor of two’ phrasing is being used loosely, in a marketing way,” he said. “Who’s going to say, ‘We boosted it by a factor of 1.7?’”

In truth, it wasn’t even 1.7.

We know many of Nissan’s engineers and executives, and they’re an upfront and honest bunch. Maybe there’s a logical explanation for this discrepancy, and we welcome it.

But from what we can tell, this sounds a little bit too much like the good old days of electric vehicles, when battery suppliers would say that that 300 Wh/kg was right around the corner, along with 15-minute recharge times and 400 miles of range. One of the reasons some people soured on EVs a decade ago was because of that kind of overstatement. The reality didn’t live up to the hype.

So here’s my suggestion for Nissan: If the energy density is actually 55% higher, why don’t you just say, “It’s 55% higher.”

It might not sound as good, but from what I can tell, it appears to be a lot more accurate.

A few weeks ago, Ford Motor Co. quietly announced that it was rolling out a new wrinkle to the powerful safety feature called stability control, adding even more lifesaving potential to a technology that has already been very successful.

A well-known automotive consultant who did an extensive teardown of BMW’s i3 all-electric car said its design is groundbreaking in multiple ways. “We’ve torn down about 450 cars, and we’ve never analyzed anything like this before.”

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